Several approaches are known in the art to generate tunable few-cycle fight pulses.
A first such method involves the four-waves mixing during the propagation of ultrashort laser pulse inside a gas-filled hollow capillary. Referred can for example be made to Canadian patent application no. 2,384,325 (KARASAWA et al.) on this subject. As shown in FIG. 1 of KARASAWA, for gas-filled hollow capillary, the optimization of the phase-matching for a given wavelength is achieved by adjusting the gas pressure inside the capillary with a gas chamber. This phase-matched gas pressure is critical for the energy conversion efficiency of the generated pulse: a small shift of the gas pressure results in severe decrease of the conversion efficiency.
With reference to U.S. Pat. No. 6,813,429 (PRICE et al) and U.S. Pat. No. 6,870,663 (KATO et al), it is also known to generate tunable light pulses through the soliton-self-frequency shifting effect in a microstructured fiber. For this technique (see FIG. 1 of PRICE), the ultrashort optical pulse propagating in the microstructured fiber experiences temporal reshaping and frequency down-shifting due to non-instantaneous and nonlinear response of the fiber's glass. In this soliton self-frequency shift process, the wavelength of the generated pulse depends on the fiber length. Moreover, the energy conversion efficiency of the generated self-frequency soliton is low because of the low coupling efficiency of the pumps and the high propagation loss due to the small fiber's core.
The energy scales of the techniques mentioned above are limited to a few hundred of nanojoules per generated pulse. Gas-filled hollow capillary and micro-structured fiber have serious inherent limiting factors: Energy scalability is limited; beam pointing fluctuations of the incoming beam directly translate into unwanted energy and pulse parameter fluctuations of the outgoing pulses and the performance critically depends on the quality of the fiber and capillary.
It is also known in the art to use a selectively tunable optical parametric generator to generate tunable light pulses, as for example shown in U.S. Pat. No. 5,144,629 (BASU) and U.S. Pat. No. 5,371,752 (POWERS et al). For optical parametric generator, such as an optical parametric oscillator (OPO) or an optical parametric amplifier (OPA), turnability is limited to a down-frequency shift of the laser pump through second order optical parametric effect in a nonlinear crystal. The group velocity mismatch between the pump and the generated pulses is significant and the spectral bandwidth of the generated pulse limits the crystal thickness to less than 1 mm thick for the generation of sub-100 fs laser pulse (see V. Petrov, F. Noack: Opt. Lett. 21, 1576 (1996)). Using a thinner nonlinear crystal for the parametric amplification results in a lower conversion efficiency. The energy of the generated pulse can reach the order of microjoule per pulse, but at the cost of complex multi-pass optical parametric amplifier setup. This is for example the case in E. {hacek over (Z)}eromskis, A. Dubietis, G. Tamo{hacek over (s)}auskas, A. Piskarskas: Opt. Comm. 203, 435 (2002). In the first stage, tunable pulses are parametrically amplified in a nonlinear crystal from a generated white-light continuum seed. The white-light continuum seed was produced by a small portion of the pump pulse that was focused in a thin glass plate to generate white-light continuum by self-phase modulation. In the second and third stages, a pump beam for each amplification stage was produced by means of a 50% beam splitter. The pump beams were directed to the nonlinear crystal and the generated pulse was amplified in the nonlinear crystal if the temporal and spatial superposition of the pump beams with the white-light continuum seed was achieved by using the delay matching lines. There is therefore a need for an improved versatile approach to generating light pulses having desired optical characteristics.